Modern electronics conventionally uses an oscillator for a variety of reasons, including producing a clock signal. One method of producing an oscillating signal includes connecting an odd number of inverting delay stages in series and providing the output of the last inverting delay stage to the input of the first inverting delay stage. Because the number of delay stages is odd, the output of such an oscillator cycles from high to low and back to high with a period equal to the delay of each delay stage multiplied by two times the number of delay stages. Thus, the frequency of such an oscillator is the reciprocal of the product of the delay of one stage and twice the number of stages.
If the above-described device produces an oscillating signal oscillating between a high value and ground, the device is referred to as a single-ended oscillator. Single-ended oscillators are desirable because they are relatively immune to substrate and thermal noise. However, they are more susceptible to power supply noise than a differential oscillator. Therefore, a single-ended oscillator is particularly desirable in digital applications.
A conventional approach to controlling the frequency of single-ended oscillators is to increase the frequency of the oscillator by increasing the drive strength of an inverter in a delay stage of the oscillator. This has been accomplished by increasing the number of inverters in parallel or by attaching a controlled amount of capacitance, through a voltage controlled resistor, to the output node of each stage. Although valid, these approaches have disadvantages. First, the silicon area required to produce such an oscillator can be tremendous. Second, the control versus range is not very linear.